# Templates

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A template is a C++ entity that defines one of the following:

- a family of classes (class template), which may be nested classes
- a family of functions (function template), which may be member functions

an alias to a family of types (alias template) | (since C++11)

a family of variables (variable template) | (since C++14)

a concept (constraints and concepts) | (since C++20)

Templates are parameterized by one or more template parameters, of three kinds: type template parameters, non-type template parameters, and template template parameters.

When template arguments are provided, or, for function and class(since C++17) templates only, deduced, they are substituted for the template parameters to obtain a specialization of the template, that is, a specific type or a specific function lvalue.

Specializations may also be provided explicitly: full specializations are allowed for class, variable(since C++14) and function templates, partial specializations are only allowed for class templates and variable templates(since C++14).

When a class template specialization is referenced in context that requires a complete object type, or when a function template specialization is referenced in context that requires a function definition to exist, the template is instantiated (the code for it is actually compiled), unless the template was already explicitly specialized or explicitly instantiated. Instantiation of a class template does not instantiate any of its member functions unless they are also used. At link time, identical instantiations generated by different translation units are merged.

The definition of a class template must be visible at the point of implicit instantiation, which is why template libraries typically provide all template definitions in the headers (e.g. most boost libraries are header-only).

### Syntax

`template <`parameter-list﻿`>` requires-clause﻿(optional) declaration | (1) |
`export template <`parameter-list﻿`>` declaration | (2) | (until C++11)
`template <`parameter-list﻿`> concept` concept-name `=` constraint-expression﻿`;` | (3) | (since C++20)

parameter-list | - | a non-empty comma-separated list of the template parameters, each of which is either non-type parameter, a type parameter, a template parameter, or a parameter pack of any of those(since C++11).
requires-clause | - | (since C++20) a requires-clause that specifies the constraints on the template arguments.
declaration | - | declaration of a class (including struct and union), a member class or member enumeration type, a function or member function, a static data member at namespace scope, a variable or static data member at class scope(since C++14), or an alias template(since C++11). It may also define a template specialization.
concept-name constraint-expression | - | see constraints and concepts

export was an optional modifier which declared the template as exported (when used with a class template, it declared all of its members exported as well). Files that instantiated exported templates did not need to include their definitions: the declaration was sufficient. Implementations of export were rare and disagreed with each other on details. | (until C++11)

| This section is incomplete Reason: core syntax, template parameters, and instantiations, take content common between class_template and function_template

### Template identifiers

A template identifier has one of the following syntaxes:

template-name﻿`<`template-argument-list﻿(optional)`>` | (1) |
`operator` op﻿`<`template-argument-list﻿(optional)`>` | (2) |
`operator ""` identifier `<`template-argument-list﻿(optional)`>` | (3) | (since C++11) (deprecated)
`operator` user-defined-string-literal `<`template-argument-list﻿(optional)`>` | (4) | (since C++11)

1) A simple template identifier.
2) An operator function template identifier.
3,4) A literal operator function template identifier.

template-name | - | an identifier that names a template
op | - | an overloadable operator
identifier | - | an identifier
user-defined-string-literal | - | "" followed by an identifier

A simple template identifier that names a class template specialization names a class.

A template identifier that names an alias template specialization names a type.

A template identifier that names a function template specialization names a function.

If all following conditions are satisfied, a template identifier is valid﻿:

- There are at most as many arguments as there are parameters or a parameter is a template parameter pack(since C++11).
- There is an argument for each non-deducible non-pack(since C++11) parameter that does not have a default template argument.
- Each template argument matches the corresponding template parameter.
- Substitution of each template argument into the following template parameters (if any) succeeds.

If the template identifier is non-dependent, the associated constraints are satisfied as specified below. | (since C++20)

An invalid simple template id is a compile-time error, unless it names a function template specialization (in which case SFINAE may apply).

    template<class T, T::type n = 0>
    class X;

    struct S
    {
        using type = int;
    };

    using T1 = X<S, int, int>; // error: too many arguments
    using T2 = X<>;            // error: no default argument for first template parameter
    using T3 = X<1>;           // error: value 1 does not match type-parameter
    using T4 = X<int>;         // error: substitution failure for second template parameter
    using T5 = X<S>;           // OK

When the template-name of a simple template id names a constrained non-function template or a constrained template template parameter, but not a member template that is a member of an unknown specialization, and all template arguments in the simple template id are non-dependent, the associated constraints of the constrained template must be satisfied:
    template<typename T>
    concept C1 = sizeof(T) != sizeof(int);

    template<C1 T>
    struct S1 {};

    template<C1 T>
    using Ptr = T*;

    S1<int>* p;                      // error: constraints not satisfied
    Ptr<int> p;                      // error: constraints not satisfied

    template<typename T>
    struct S2 { Ptr<int> x; };       // error, no diagnostic required

    template<typename T>
    struct S3 { Ptr<T> x; };         // OK, satisfaction is not required

    S3<int> x;                       // error: constraints not satisfied

    template<template<C1 T> class X>
    struct S4
    {
        X<int> x;                    // error, no diagnostic required
    };

    template<typename T>
    concept C2 = sizeof(T) == 1;

    template<C2 T> struct S {};

    template struct S<char[2]>;      // error: constraints not satisfied
    template<> struct S<char[2]> {}; // error: constraints not satisfied
| (since C++20)

If all following conditions are satisfied, two template identifiers are same﻿:

- Their template-name﻿s or operators refer to the same template.
- Their corresponding type template arguments are the same type.
- The template parameter values determined by their corresponding non-type template arguments are template-argument-equivalent.
- Their corresponding template template arguments refer to the same template.

Two template identifier that are the same refer to the same variable,(since C++14) class, or function.

### Templated entity

A templated entity (or, in some sources, "temploid") is any entity that is defined (or, for a lambda expression, created)(since C++11) within a template definition. All of the following are templated entities:

- a class/function/variable(since C++14) template

a concept | (since C++20)

- a member of a templated entity (such as a non-template member function of a class template)
- an enumerator of an enumeration that is a templated entity
- any entity defined or created within a templated entity: a local class, a local variable, a friend function, etc

the closure type of a lambda expression that appears in the declaration of a templated entity | (since C++11)

For example, in

    template<typename T>
    struct A
    {
        void f() {}
    };

the function `A::f` is not a function template, but is still considered to be templated.

A templated function is a function template or a function that is templated.

A templated class is a class template or a class that is templated.

A templated variable is a variable template or a variable that is templated. | (since C++14)

### Keywords

`template`, `export`

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
CWG 2293 | C++98 | the rules of determining whether a template identifier is valid were not provided | provided
CWG 2682 | C++98 C++14 | the definitions of templated function/template class (C++98)/templated variable (C++14) were missing | added
P2308R1 | C++98 | two template identifiers were different if their corresponding non-type template arguments are not template-argument-equivalent | they are different if their corresponding non-type template parameter values are not template-argument-equivalent

### See also

C documentation for Generic selection

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